This invention relates to well completions in which a perforating gun and a packer are run into a well together on a string of tubing, with the perforating gun detonator initially sealed off from the tubing string by a frangible disc, with a vent between the disc and packer to allow formation fluid to enter the tubing after the formation is perforated. The gun may be released and dropped to bottom.
The completion of wells by employment of permanent completion devices is known to those skilled in the art as evidenced by the following Vann U.S. Pat. Nos.: 3,706,344; 3,871,448; 3,966,26; and 4,066,282. In carrying out permanent completion techniques, it is advantageous to employ packer actuated vent assemblies and perforating guns such as disclosed in the above-mentioned patents. Various different packers, including permanent packers, are known such as described, for example, in the 1974-1975 Baker Catalog, Section III, IV, and V (Baker Oil Tools, Inc., P.O. Box 3048, Houston, Tex. 77001).
After a borehole has been drilled into the ground and the casing cemented into position, a considerable amount of time may expire before the well is completed and suitably connected into a gathering system. When a tool string is run into the well open, the well fluid fills the tool string with drilling mud and debris. When such a string of tools has been left downhole for a period of time, the mud and debris begin to gravitate towards the lower end of the string and begin to densify and congeal into a heavy layer of material. The longer the tool string is left downhole, the more mud is permitted to settle and congeal.
Such contamination of the interior of the tool string can also occur even when the tool string is run into the well closed such that the drilling mud cannot flow into the tubing string. Debris inside the tubing string, such as flakes, rust, sand, scale and other material dropped into the well from the surface, tends to collect in the bottom of the string. Often such debris becomes dislodged and falls down into the tubing string as the tubing string is handled and lowered into the well. Again, the heavy particles and other suspended matter will gravitate to the bottom of the tool string where such contaminants densify into a heavy layer of material. Thus, even though there is no drilling mud which settles and congeals, debris still may tend to collect and settle in the string.
In a perforating gun having a bar actuated gun firing head for example, it is possible for such contaminants to densify and collect about the gun firing head mechanism and become so compacted and viscous that the gun firing head cannot be sufficiently impacted to detonate the perforating gun. The firing mechanism generally requires 20 ft-lbs of impact for detonation. The debris can be so compacted and packed that the bar may be unable to sufficiently strike the firing mechanism with 20 ft-lbs of force. Thus, a misfire could occur because of the accumulation of debris that enters the tubing string and contaminates th firing mechanism. The problem of contamination is of course compounded when the perforating tool string is left downhole for a substantial length of time.
The amount of debris which will prevent detonation will vary depending upon how hard the debris is packed over the firing mechanism. Even an inch or two of debris, if packed down, may prevent a sufficient impact of the bar to detonate the gun.
The well completion method and apparatus of the present invention is particularly applicable in wells having a depth, temperature, and formation pressure which will permit the use of a retrievable packer mounted on a tubing string for supporting a perforating gun within the well. The present invention discloses two different embodiments, one embodiment including an open system where the tool string is run into the well wet, i.e., the drilling mud is permitted to flow into the tubing string, and another embodiment for a closed system, i.e., the drilling mud is not permitted to flow into the tubing string.
Accordingly, it is desirable to have a perforating tool string which is arranged in a manner whereby the lower borehole annulus is isolated from the passageway immediately above the gun firing head. The elimination of the contaminants from the lower tubing string has been found to greatly enhance the operation of the perforating gun. The present invention prevents the mud and debris from collecting around the firing mechanism by isolating the firing mechanism in a fluid tube. Further, it is advantageous to not only isolate the firing mechanism, but also easily permit access to the gun firing head upon detonation. The method and apparatus for accomplishing these purposes is the subject of the present invention.